I tried to start Firefox, Mozilla, or Seamonkey, but got an error message saying that it's already running, and I don't have it open. What's going on?

The Mozilla family of browsers checks to see if another process is already using a profile by looking for the existence of a file named .parentlock in your profile. If a browser process dies or otherwise fails to exit cleanly, it can sometimes leave this lock file behind (the lock file should be present while the browser is running).

In Linux, this file will exist at ~/.mozilla/$BROWSER/$PROFILE/.parentlock, in OSX it exists at ~/Library/Application Support/Firefox/Profiles/$PROFILE. $PROFILE usually takes the form of eight random characters followed by a ‘.’ and the string ‘default’ or ‘Default User’.

On Linux you may locate this file by hand (cd; ls -Ra .mozilla | less, use ‘/’ to enter a search query, look for .parentlock), or with the following command (NOTE: This will remove all instances of .parentlock in your .mozilla directory. Do not use this command without first exiting all Mozilla- family browser sessions on any machines you may be logged into.):

cd; find .mozilla -name \.parentlock -exec rm -v {} \;

On Mac OS X you may locate this file by hand (ls -Ra ~/Library/Application\ Support/Firefox/Profiles/ | less, use ‘/’ to enter a search query, look for .parentlock), or with the following command (NOTE: This will remove all instances of .parentlock in your .mozilla directory. Do not use this command without first exiting all Mozilla-family browser sessions on any machines you may be logged into.):

cd; find ~/Library/Application\ Support/Firefox/Profiles/ -name \.parentlock -exec rm -v {} \;